This disclosure relates to a process for repairing damaged portions of a metal component and to a repaired metal component. More particularly, it relates to a process for repairing turbine components and to repaired turbine components.
Certain components of a power generating apparatus, for example a turbine engine, operate in a hot gas path of the apparatus. In the turbine section of a gas turbine engine, components are often subjected to significant temperature extremes and contaminants present in combustion gases resulting in cyclic loading from thermal expansion as well as wear from the hot gas flow. As a result of operating in such an environment, exposed portions of the turbine components are subject to degradation. Various forms of degradation may include, but are not limited to, oxidation effects, crack formation, and/or erosion and wear, such as on the airfoil and sidewall surfaces of the turbine component. These types of degradation can lead to a loss in turbine operating efficiency and possible damage to components located downstream from the degraded portion as a result of separation and contact therewith.
Within the aerospace and power generation industry, it has become much more economical to repair costly engine components than to replace them. The repair and restoration of the damaged portion prevents catastrophic failure, improves efficiency of the overall gas turbine, and reduces operating costs. To provide sufficient repair and restoration, the process should be able to restore the surface to its original dimensions while providing a structurally sound structure. Moreover, the duration of the repair should be minimal. To effect repair and restoration, the entire damaged portion is typically removed, e.g., by milling, cutting, machining, gouging, laser ablation, or the like, so that a compatible filler material can be deposited or coated. The filler material is then deposited to replace the removed portion. Adequate adhesion between the filler material and the parent or base metal is an important consideration for the repair. Ideally, the filler material should provide similar physical and mechanical properties as the parent metal. In addition, because of the harsh operating conditions to which the filled material and turbine component will be exposed, it is desirable that the filler materials exhibit sufficient corrosion and environmental resistance. It is also desirable that the processes for repairing the turbine component not damage the areas surrounding and extending beyond the repaired portion.
Current repair processes typically include a weld repair of the individual cracks, if visible, or replacement of the damaged portion by welding a coupon of new material into the recess formed by the removed damaged portion. Generally, welding repairs cracks or joins the coupon of new material to the parent component by melting and fusing them together. In order to fuse the metals, a concentrated heat source is applied directly to the joint area. This heat source is high temperature in order to melt the parent metal and the filler metals (the metals being joined). Because welding heat is intense, it is impractical to apply it uniformly over a broad area.
Unfortunately, because of the non-uniform concentrated heating and high temperatures employed, welding processes can cause a significant distortion to the turbine component to be repaired due to the severe thermal gradients across the turbine component caused by the non-uniform concentrated heating; can deleteriously lock in internal stress; can result in the formation of secondary cracks; can present difficulties repairing damaged portions proximate to cooling holes in the turbine component (e.g., such as nozzles) without affecting the holes themselves; and can require a significant amount of time and manual labor to effect the repair.
In addition to these problems that can be caused by welding, a preheating step may be required under certain circumstances as part of good welding practice. The purpose of the preheating step is to prevent hydrogen-induced cracking; this type of cracking occurs after the weld has cooled and usually runs from the toe of the weld or from other weld defects. Such a crack is difficult to detect and can be detrimental to the service life of the turbine component, particularly if the welds are located in highly stressed areas. The increased temperature from preheating increases the diffusion of hydrogen and bakes the hydrogen out of the weld. Preheating also allows a slower cooling rate of the weld preventing excessive loss of ductility in the weld and the heat-affected zone of the base metal. Of course, preheating also adds significant expense and repair times for the turbine component.
Also, in those cases where the welding process introduces distortion, the turbine component may need to be subjected to mechanical forces and/or a heat treatment to eliminate the stress caused by the distortion. Turbine components are of exacting specifications and it is important that the original shape and original specifications be maintained after the repair and restoration is complete.
Accordingly, there remains a need for improved repair and restoration processes for damaged turbine components.